Explanation: Galaxies are fascinating not only for
what is visible, but for what is invisible.
Grand spiral galaxy
NGC 1232,
captured in detail by one of the new
Very Large Telescopes,
is a good example.
The visible is dominated by millions of
bright stars and dark
dust,
caught up in a gravitational swirl of
spiral arms revolving about the centre.
Open clusters containing
bright blue stars can be seen sprinkled along these spiral arms,
while dark lanes
of dense interstellar
dust
can be seen sprinkled between them.
Less visible, but detectable, are billions of dim normal
stars and vast tracts of
interstellar gas,
together wielding such high mass that they
dominate the dynamics of the inner
galaxy.
Invisible are even
greater amounts of matter
in a form we don't yet know - pervasive
dark matter needed to explain the
motions
of the visible in the outer galaxy.